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Curiosity’s Martian Descent, 7 Minutes Of U. S. Space Excellence

By Craig Covault

This graphic highlights what is known as the “seven minutes of terror” – the period where Curiosity is on its own as it goes through the most dangerous part of its mission. Image Credit: Max-Q Entertainment

Nearly 154 million mi. from Earth, the Mars Science Laboratory (MSL) rover Curiosity is poised to meld the talents of 7,000 U. S. space workers from 37 states into 7 minutes of genuine American technology leadership as Curiosity makes a daring descent to the Martian surface to unravel the potential for life on another planet.

The nearly 2,000 lb. NASA rover, built with major contributions from Spain, Canada, France, Germany, Russia and the European Space Agency is set for a 10:31 PDT August 5 landing inside a 12 mi. long by 4 mi. wide mountainside ellipse thousands of feet deep in Gale Crater.

Jet Propulsion Laboratory Mission Controllers at the famed facility in Pasadena, Calif. are likely to command MSL to fire its maneuvering thrusters today or Saturday in one final trajectory correction maneuver (TCM) to cancel out as much as 3,000 ft. of remaining targeting error as the final approach nears.

Trajectory that MSL will follow over the Martian surface, flying east showing the point where the entry will begin, auto guidance initiated and parachute deployed over Gale Crater. in relation to the MRO relay’s orbital path. Image Credit: NASA JPL

MSL is now within 500,000 mi. of Mars flying at 3,800 mph as it nears the end of its 354 million mi. curving trajectory that began with launch last Nov. 26 from Cape Canaveral, on an Atlas V rocket . A larger TCM maneuver July 28 moved MSL by 13 mi. to bring it nearly onto the final approach path except for the final tweak as Mars looms ahead.

The pull of Martian gravity will accelerate MSL to 13,200 mph, at entry interface when MSL will experience 10gs during the first moments of the descent as Martian atmosphere begins to rapidly slow the vehicle. This begins JPL’s “7 Minutes Of Terror as portrayed in what has become an internationally popular YouTube Video.

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Video courtesy of NASA

But MSL engineer Steven W. Sell, speaking in Spain before a planetary workshop explained that rather than being a seat- of -the –pants approach, the descent is focused on a highly refined methodology including:

–A strategy that also “implements a mid point correction guidance style where errors are corrected at distinct points in the descent called accordions which can be bounded by ‘error budgets’ “to prevent the propagation of greater errors downstream in the landing sequence.

–And if errors do occur “the implemented style allows for a ‘graceful degradation’” that can be caught and corrected .

But MSL Project Manager Pete Theisinger who I have interviewed often about MSL challenges over the last eight years noted “The 7 min. of terror are for the non predictable elements of the descent.”

Graphic shows the original trajectory prior to July 28 maneuver (green line) that moved the trajectory 13 mi. south (yellow line) for most precise flightpath to Gale Crater. Later analysis by JPL showed another correction maneuver was likely to take out several hundred feet of additional error. Image Credit: NASA JPL

“Fly as you test is JPL’s tried and true mantra.” he said. But he also stressed that Entry, Descent and Landing (EDL ) “is a classic example that you cannot do everything on Earth.”

“We did a whole bunch of testing, but we have never fired all 76 pyros in 7 min. in an atmospheric environment like Mars starting at 10gs. We have not done anything like that,” said Theisinger.

We have analyzed the hell out of it, to see if there is something we have missed, some subtlety, some unplanned interactions. For example the new electronics works at lower and lower voltages. Does that make a difference to us?”

“We have tried to answer those questions, but they are never actually answered until we actually fly the system in the Martian environment. That is the terror part of this,” Theisinger said, becoming choked up with emotion as we completed our discussion.

Mars Science Laboratory Curiosity rover at Cape Canaveral shortly before its Nov. 26, 2011 launch show how it will look sitting on Mars after a successful landing late Aug. 5 Pacific time. Silver can on back is antenna to relay satellites, Mast with ChemCam laser and multiple cameras stands 7 ft. tall and front of rover has spare drill bits and rust colored inorganic carbon test blanks. Arm extends to extreme right. Photo Credit: Jeff Seibert

Martian weather is also critical as winter gives way to spring at the Gale Crater area slightly south of the equator. In Earth terms, the landing site would be positioned just north of Australia.

The MSL Deputy Project Scientist Ashwin Vasavada has been the lead manager for watching both the planet’s own weather as well as solar weather created by the Sun. The winds are holding to MSL’s predicted wind models, as determined by the Mars Reconnaissance Orbiter‘s measurement of surface temperature in the landing site region.

There is a dust storm about 600 mi. south of the landing site that theoretically could spread some dust into the Gale Crater atmosphere. But Vasavada says dust storms this time of year normally last for only about two days which means this one should be gone by landing time.

“MSL is very, very complicated,” says Rob Manning Curiosity chief engineer. “ It is almost hard to imagine how complex it is,” he said.

While the Sky Crane landing system draws the greatest attention, it is the coupling of active guidance with aerodynamics to fly–rather than fall– in the Martian atmosphere that will be the key to MSL’s ability to make a precise landing.

Curiosity has far more computing capability than the MER Mars Exploration Rovers Spirit and Opportunity that each had only a single, much less capable computer. Curiosity has two BAE RAD 750 computers with a PowerPC 750 architecture that operates at up to 200 MHz speed 10 times faster than the MER rovers single processor. Each of Curiosity’s 2 computers also has 2 gigabytes of flash memory, 8 times more than the MERs.

MSL’s parachute has a direct heritage to past Mars missions, but its thrusters and landing engines are the oldest component designs on the spacecraft. Its eight Mars landing engines go back to Viking in 1976 and more than 320 engines from MSL’s two main thruster groups have already flown in space going back 30 years over the course of dozens of space missions. But everything else on MSL involved in landing is cutting edge new.

The guided reentry, landing radar and Sky Crane have undergone extraordinary computer and physical hardware tests including millions of simulated Mars landings in powerful computers at the NASA Langley Research Center, Hampton, Va. , said David Way, the Langley engineer leading the simulations.

This precision entry and landing, coupled with the Sky Crane delivery system will give NASA a new workhorse Mars landing system for use by future landers and rovers that, like Curiosity, are 2,000 lb. or heavier, says Doug McCuistion, Mars Program Director at NASA Headquarters. Future manned Mars operations may require precise landings of 40-100 ton payloads.

Five times heavier than Spirit or Opportunity, the MSL rover is “by far” the most ambitious spacecraft ever developed by JPL said Rob Manning Curiosity chief engineer.

The eight Aerojet Mars Landing Engines on the Sky Crane are paired. All eight fire initially then only four are used for final descent. The engines have a design identical to the 1976 Viking lander engines that used a “shower” nozzle as opposed to these more traditional nozzles. Photo Credit: Alan Walters / awaltersphoto.com

“It is difficult to overstate what a major step forward this is beyond the earlier Mars Exploration Rovers” says Thesinger.

Curiosity’s computer using atmospheric entry algorithms from the Apollo program will steer by ejecting heavy tungsten ballast at two different times during atmospheric entry. This will alter the vehicle’s center of gravity, enabling the MSL aeroshell to fly at differing angles of attack.

It will also perform left and right hypersonic bank maneuvers at up to about 70 deg. of bank to change its lift vector to maneuver left and right of the centerline to Gale Crater. As with Apollo and space shuttle orbiters, these roll reversals will adjust it’s range and descent rate to precisely navigate to its touchdown area.

MSL is also more than twice the physical size of the Mars Exploration Rovers (MERs) Spirit and Opportunity rovers that landed in 2004, and Curiosity carries 150 percent more science payload mass.

The 14.8 ft. dia heat shield for the MSL aeroshell is two feet larger than an Apollo command module heat shield. The MSL heat shield under a silver cover here must be shed immediately after parachute deploy so the rover radar heads can see the Martian surface. Photo Credit: Alan Walters / awaltersphoto.com

Another key to accuracy and a safe landing will be a new radar configuration never flown to Mars before. “The radar has taken substantially more time to develop than desired, but it is a superb radar,” says Theisinger.

“We needed good velocity and altimetry data relative to surface of Mars. When slowing from nearly 4 mi. per sec. It is tough to get the velocity data correct down to under a meter per sec.—and that is what we need to land this thing,” said Adam D. Steltzner, JPL Entry, Descent and Landing manager for MSL.

“We feel great about the radar,” Steltzner said. “One reason we decided to build our own radar is that we struggled in the past with Phoenix, MER and Pathfinder radars when we tried to modify existing weapons system radars,” he said.

During its 9 month curving trajectory from Cape Canaveral to Mars the aeroshell with Sky Crane and rover inside are flying attached to a large disk shaped cruise stage with solar cells on top. During cruise the aeroshell will have a symmetrical mass.

The 14.8 ft. dia aeroshell for MSL carries several hundred pounds of tungsten mass to alter the spacecraft’s center of gravity to use the aeroshell’s lift steer MSL to the landing site. Large plates of tungsten are under hatch opening for insertion of nuclear power source into rover before launch. Photo Credit Alan Walters / awaltersphoto.com

Separation of the cruise stage will occur 10 min. before the start of the entry. Just after cruise stage separation and before entry 330 lb. of ballast will be ejected.

“That will make us fly at a canted angle that will enable the heat shield to develop lift. Steering and lift control will be achieved by the rover computers calculating when to fire attitude control jets to roll or bank the vehicle.

For the first time in any Mars landing, the attitude and velocity of the vehicle will be used continuously in a closed loop data stream for real time maneuvering commands to landing within the small landing ellipse.

Oblique view of Sky Crane shows three pair of MLEs and the vehicle’s wide stance under which to place the rover. Photo Credit Alan Walters / awaltersphoto.com

The untested skycrane still must work perfectly after the high and fast portion of the reentry brings MSL into the Gale crater area. Here is how that will happen:

–Second ballast ejection: Just before parachute deploy another 330 lb. of tungsten ballast will be separated, this time to relocate the center of gravity back to the middle of the vehicle to provide a balanced condition for the rest of landing events.

—Supersonic Parachute deploy: Velocity measurements will deploy the single Pioneer Aerospace 52.5 ft. dia. parachute when the vehicle has slowed to 1,000 mph at about 6 mi. altitude. On MSL the chute will be lowering a mass of 3,400 lb. including the aeroshell, Sky Crane and rover.

—Heat shield separation: After the parachute opens the heat shield will be jettisoned. This opens the bottom of the aeroshell and cues activation of the landing radar with transmit and receive heads on a bat shaped tray extending off the side of the rover.

—Real time imaging: The Mars Decent Imager attached near the lower side of Curiosity will begin taking a continuous stream of high resolution images at up to 4 /sec. Developed by Malin Space Science Systems, the imager will look down on the terrain, showing it grow closer and closer during the landing.

—Backshell and parachute separation: Descending through 6,000 ft. at 180 mph the vehicle will separate its backshell and parachute revealing the Mars Sky Crane and Mini Cooper sized rover.

The first thing the Sky Crane and Curiosity do is nothing. The vehicle is programmed to freefall for 1 sec. to be well clear of the 100 ft. long parachute canopy, risers and backshell.

Paired on each corner of the Sky Crane , all 8 MLEs are ignited as the Sky Crane and rover streak through 6,000 ft. at 180 mph. Engine ignition will dramatically slow the descent and gain attitude control for the fast approaching Martian touchdown.

The first thing the vehicle does is maneuver laterally 1,000 ft. to prevent the disaster of having the backshell and parachute collide in midair or accidentally drop onto what had been a safely landed rover.

The Sky Crane still holding Curiosity tight, will begin to descend at 71 mph for about 3 sec. It will then slow to a sedate 7 mph descent rate.

After the lateral maneuver the sky crane’s thrusters will null out motions in all axis that may be left over from the separation of the parachute. The radar will temporarily stop looking for the ground and shift to velocity measurements to establish this rate.

During this period, the rover will be flying in one of three “ terrain accordions” where there will be ample room for the computers to command major adjustments in altitude and velocity.

When the engines have slowed the descent to 1.7 mph, four of the 8 MLE’s are throttled to only 1% as MSL holds that slow velocity until touchdown. The other four are throttled as needed in the 30-50% power range.

As the descent continues to 66 ft. altitude the rover is released by the Sky Crane and begins to be lowered on its triple line 60 ft. bridle (BUD) just 12 sec. from touchdown. The rover will unfold its six wheeled rocker bogie mobility system midway in the descent.

The scene will be of endless red terrain, with a looming mountain in the middle of giant Gale crater below, as the two vehicles now fly for several seconds as two distinctly separate vehicles connected by the BUD cords .

The rover will drop more rapidly on the bridal than the Sky Crane is descending. As it reaches the surface the rover computer will sense the Sky Crane trying to accelerate upward, since it just lost 1 ton of payload mass. This will cue Curiosity to open latches, severing the three connections holding the rover onto the bridal. It will also cue the Sky Crane to begin its flyaway maneuver to fly out of the area and crash about one half mile away.

“We love to smartly say that we do not look for the touchdown event, but rather perceive the post landing state of the vehicle” Steltzner said. It will be about 3 p.m. at the Gale crater site. The touchdown will occur on Sol zero of the flight.

With Curiosity on the ground a whole new set of differences between Spirit and Opportunity and the new MSL rover become apparent.

–Wheels and speed: Curiosity will use six wheels each 50 cm in diameter. This compares with the 20 cm wheels for Spirit and Opportunity. The larger wheels will give Curiosity a 20% increase in max speed to 6 cm/sec. compared with Spirit and Opportunity. It larger wheels will enable curiosity to climb over larger rocks, up to 29 in. high.

—-Size comparison: Curiosity (below) is 8.8 ft. wide, 7.2 ft. tall and 9 ft. long compared with the two MER rovers that each were 7.5 ft. wide, 4.9 ft. high and 5.2 ft. long.

Will Curiosity herald a new era in Martian exploration – or will her mission end in the early-hours on Monday? Tune into AmericaSpace to find out. Photo Credit: Alan Walters / awaltersphoto.com

About the author

Craig Covault

Senior writer Craig Covault has covered 17 U. S., Russian and European Mars orbiter and lander missions over the last 40 years, most of them with Aviation Week & Space Technology and now with AmericaSpace.

Favorites include his coverage at the NASA Langley Research Center of the development of the 1976 Viking 1 and 2 landers and the landings and surface coverage from the JPL of the Spirit and Opportunity rovers in 2004. Steve Squyres Mars Exploration Rover Principal Investigator allowed Covault exclusive JPL access to the science and surface operations teams after Spirit and Opportunity landed.

Covault also did extensive coverage of the Phoenix North Polar Lander development and its descent to the Martian arctic in 2008.

Covault began interviews and hardware familiarization with Mars Science Laboratory Rover and Sky Crane developments at JPL starting in 2004 and was able to see the Curiosity and the Sky Crane hardware take shape throughout the development including an initial look a Curiosity when it was just a block of aluminum with holes being milled into it.

AmericaSpace and The Mars Society have partnered to provide in-depth coverage of the arrival of the Mars Science Laboratory rover “Curiosity” to Mars. Stay tuned for regular updates as AmericaSpace correspondents Craig Covault and Frank O’Brien travel to NASA’s Jet Propulsion Laboratory in California for live coverage.

It will be an event that will mark a turning point in the exploration of the planets of our Solar System.
The complex machine technology embedded in Curiosity is the demonstration of cognitive ability, scientific and technical man.
Now we just have to wait for and then start this new adventure.
Good job to all the staff who is leading Curiosity. Good luck!

AmericaSpace Images

Image of the Curiosity rover at NASA's Kennedy Space Center in Florida just prior to the rover's launch in November of 2011.

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